Birds of a different feather can flock together when members of one species come to recognize individuals from another, says a pioneering study from the University of Nebraska-Lincoln and University of Chicago.

The study, which yielded the first experimental evidence of recognition across non-primate species, took place in an arid region of southern Australia that is home to both variegated and splendid fairy-wrens.

Researchers recorded the songs of both species from fairy-wrens residing within the shared territory, just outside that territory or well beyond it: co-residents, neighbors and outsiders, respectively. When the team played those recordings to dominant males of the other species — a variegated song to a splendid fairy-wren, or vice versa — it found that the fairy-wrens tolerated the songs of co-residents but responded with immediate aggression to the songs of neighbors and outsiders.

The finding suggests that members of the two species, which do not migrate and may live alongside each other for years, can distinguish long-term companions from passersby or potential trespassers. By contrast, ecologists have generally assumed that even beneficial interactions between species occur without any regard for the individuals taking part in them.

“When people are considering or studying these mixed-species associations, it may be important to consider that there is an individual component to the relationships,” said Allison Johnson, the study’s lead author and postdoctoral researcher in biological sciences at Nebraska. “It’s not just: Species A benefits from the presence of Species B, or Species A is competitive with Species B.

“There are reasons to recognize (individuals) within species: kinship, reciprocal altruism, competition. It makes sense that recognition would transfer to another species if you have those same sorts of territorial or cooperative interactions.”

The researchers proposed that differentiating among members of another species, and forming long-term partnerships with those known members, may have arisen because it makes the exchange of relevant information more efficient and reliable. That, in turn, could facilitate the cooperative behavior observed in many ecosystems — jointly mobbing predators or mutual competitors, for instance. And it could help prevent each species from wasting time and energy competing with a potential ally.

Johnson and her colleagues likewise witnessed examples of cooperation between the variegated and splendid fairy-wrens, with members of both species traveling and foraging for food together. The team further discovered that the variegated fairy-wrens, which tend to live in larger groups, spent less time watching for predators and had more success rearing their chicks when familiar splendid fairy-wrens were nearby.

“It’s almost as though they’re treating (those) splendids like another group member,” said Johnson, who authored the study with the University of Chicago’s Stephen Pruett-Jones and Christina Masco.

Johnson said the findings could have implications for the conservation of species that associate with others in their territories, especially given that there is “plenty of evidence that the presence of one species can facilitate the presence of another.” One question raised by Johnson: If such a species undergoes substantial turnover in its population, could that turnover — and the eventual absence of familiar faces — affect the other species?

“From year to year, birds might come to a site that’s perfectly viable,” Johnson said. “But maybe Bob isn’t there, so they think, ‘I don’t know about this,’ and they go someplace else. If there are long-term interactions between specific individuals, that might have an impact on decision-making in terms of where they choose to breed and whether or not they return to the same site.

“It would be interesting to see if that’s true for migratory species, as well. Migration tends to re-sort individuals from year to year, so they may be willing to accept associating with whoever is there that year, but we suspect that more cross-species recognition is occurring than has been realized.”

The team’s study appeared in the journal Behavioral Ecology. Johnson, Pruett-Jones and Masco received funding in part from the National Science Foundation, the American Ornithological Society and the Norman Wettenhall Foundation.